1. Technical Field
The present invention relates to a MIMO transmission apparatus and MIMO transmission method for transmitting SC-FDMA signals in particular.
2. Description of the Related Art
In 3GPP-LTE, an SC-FDMA (Single-Carrier Frequency Division Multiple Access) is adopted as an uplink communication scheme (see Non-Patent Literature 1). With SC-FDMA, N symbols modulated by a predetermined modulation scheme (e.g., QPSK) on the time axis are separated in a plurality of frequency components, mapped on different subcarriers between frequency components and, after being changed back to a time domain waveform, attached a CP (Cyclic Prefix), thereby forming an SC-FDMA symbol. That is, one SC-FDMA symbol includes N time continuous signals and CP.
Also, in 3GPP-LTE, a radio communication base station apparatus (hereinafter simply “base station”) allocates resources for uplink data to a radio communication terminal apparatus (hereinafter simply “terminal”) via a physical channel (e.g., PDCCH).
Upon receiving information about allocation of the resources for uplink data, the terminal transmits data stored in its buffer using these resources.
Also, in 3GPP-LTE, ARQ (Automatic Repeat reQuest) is applied to downlink data from a base station to a terminal. That is, the terminal feeds back a response signal indicating the error detection result of downlink data to the base station. The terminal performs CRC (Cyclic Redundancy Check) check of the downlink data and feeds back an ACK (ACKnowledgment) when CRC=OK (no error) or feeds back a NACK (Negative ACKnowledgment) when CRC=NG (error present), to the base station as a response signal.
Therefore, a case occurs where, while transmission data stored in a buffer, the terminal has to transmit a response signal (i.e., ACK/NACK signal) for downlink data received four subframes before. In this case, on the terminal side, an ACK/NACK signal is placed on resources on which uplink data should be originally placed (see Non-Patent Literature 2).
FIG. 1 shows slots on which an ACK/NACK signal is placed. In FIG. 1, above “N” is 12, and a CP is not illustrated for each explanation. Also, one slot is formed with seven SC-FDMA symbols. Also, normally, although an ACK/NACK signal transmitted in uplink is maximum one symbol in one subframe, to increase the reception power of an ACK/NACK signal in a base station on the receiving side, an ACK/NACK signal is repeatedly transmitted eight times (i.e., eight symbols) every one slot in FIG. 1. That is, an ACK/NACK signal is transmitted four times in the SC-FDMA symbol immediately before the reference signal and four times in the SC-FDMA symbol immediately after the reference signal.
Also, 3GPP LTE-advanced standardization has been started to realize faster communication. in 3GPP LTE-advanced, to realize the uplink transmission speed of about maximum 500 Mbps, a technique of uplink spatial multiplexing (i.e., MIMO: Multiple Input Multiple Output) is expected to be adopted.
FIG. 2 is a block diagram showing a configuration example of a MIMO transmission apparatus. In FIG. 2, as in 3GPP-LTE, a data signal to be spatially multiplexed is received as input in a precoding section while maintaining a single carrier characteristic. The precoding section performs a precoding operation of two items of input stream data and outputs the results to DFT sections associated with respective antennas. Also, transmitting two items of stream data at the same time is defined as “RANK 2 transmission.” By contrast with this, in 3GPP-LTE shown in FIG. 1, “RANK 1 transmission” by means of one stream is performed. Also, it is presumed that a terminal has one transmission antenna in 3GPP-LTE, and therefore precoding processing is not performed in 3GPP-LTE.
Then, the DFT sections transform input signals on the frequency axis and output a plurality of resulting frequency components to frequency mapping sections. These plurality of frequency components are mapped on appropriate frequency positions by frequency mapping sections and made time domain waveforms by IFFT sections. The time domain waveforms obtained as above are transmitted via RF circuits and antennas.